One example of the power supply device described above is shown in FIG. 23. In this power supply device, a full-wave rectification process is performed on an input power supply (a commercial power supply) 1 by a rectifying circuit 2, and a voltage on which an alternate current/direct current (AC/DC) conversion has been performed is raised to a voltage at a predetermined level by a voltage-boosting chopper circuit (a power-factor improving unit) 3. Also, the power factor of the power supply is improved, and harmonic currents are suppressed.
In this situation, the voltage-boosting chopper circuit 3 includes: a voltage-boosting choke coil (a reactor) 3a that is connected in series to the positive terminal side of the rectifying circuit 2; a reverse-blocking diode 3b that is connected in series to the voltage-boosting choke coil 3a; a switching element (e.g. an Insulated Gate Bipolar Transistor [IGBT]) 3c that is provided between the voltage-boosting choke coil 3a and the reverse-blocking diode 3b and is connected to the negative terminal side of the rectifying circuit 2; and a smoothing capacitor 3d that smoothes an output voltage.
An operation of the voltage-boosting chopper circuit 3 is to cause a short circuit with a switching operation of the switching element 3c via the voltage-boosting choke coil 3a and also to supply the voltage on which the switching operation is performed to the smoothing capacitor 3d from the reverse-blocking diode 3b so that the voltage is used as the voltage of a load 4. For example, in a case where the power supply device is applied to a compressor motor of an air conditioner, it is assumed that the load 4 includes an inverter circuit 4a and a motor 4b. 
The present applicant filed Japanese Patent Application No. 2002-158653 that is related to a controlling method for a power supply device including the voltage-boosting chopper circuit 3 described above. The invention described in this application filed earlier will be briefly explained with reference to FIGS. 24 to 26. When an AC power supply is converted into a DC-voltage to be used as a load voltage, the converted voltage is short-circuited via at least a reactor (the voltage-boosting choke coil 3a) so that the power factor is improved.
The power supply device includes: a power-supply phase detecting circuit 5 that is operable to detect a zero cross of an AC power supply 1; a current sensor 6 that is operable to detect an input current Ii of the voltage-boosting chopper circuit 3; a control unit 8 that controls the switching element 3c based on the detected values as well as an input voltage Vi and an output voltage Vo of the voltage-boosting chopper circuit 3; and a driving unit 7 that drives the switching element 3c according to a signal from the control unit 8.
The control unit 8 performs a switching operation on the switching element 3c of the voltage-boosting chopper circuit 3 and also switches on and off the switching element 3c according to a comparison result between the input current and an input-current reference signal having a sinusoidal waveform so that the output voltage Vo of the voltage-boosting chopper circuit 3 is used as the voltage of the load.
In this situation, as shown in FIG. 24, a calculating unit 8a calculates a deviation between an output-voltage instruction value and the detected output voltage Vo. Based on the calculated deviation, a current-reference-signal amplitude generating unit 8b generates an amplitude value of an input-current reference signal Ir (an amplitude value having a sinusoidal waveform and being used as a so-called reference).
A multiplying unit 8c multiplies the generated amplitude value by the detected input voltage Vi. A hysteresis comparator 8d generates a hysteresis based on the result of the multiplication, i.e., the input-current reference signal (a current instruction value) and the current detection value Ii. An upper-limit value and a lower-limit value for the input current are generated based on the hysteresis. In other words, the switching operation is performed on the switching element 3c so that the input current Ii falls in a range defined by the upper-limit value and the lower-limit value.
On the other hand, the power-supply phase detecting circuit 5 detects a zero cross of the AC power supply. Also, a switching-operation-prohibited-time generating unit 8e generates a predetermined time between a point in time that is a predetermined time before the zero cross and the zero cross. The hysteresis output is prohibited by an AND circuit 8f, based on the generated signal.
As a result, as shown in FIGS. 25 and 26, the switching operation of the switching element 3c is prohibited only during the prohibition interval. Thus, the input current is forced to be zero at the zero-cross point of the input AC power supply. Accordingly, the input AC waveform near the zero-cross point is improved (the input AC waveform becomes closer to a sinusoidal waveform), and it is therefore possible to reduce high-order harmonic currents.